1. Field of the Invention
The present invention relates to an active pixel array of a CMOS image sensor, in which a color filter array including a filter which transmits all wavelengths of light signals and a double PN junction photodiode are used to simultaneously detect two kinds of light signals, of which the wavelengths are different from each other, in one pixel and to secure a wide light receiving area, so that high resolution, a high signal-to-noise ratio (S/N), and an improved dynamic range can be expected.
2. Description of the Related Art
In a general image sensor, when light enters a photoconductor through a color filter, the electron-holes generated in the photoconductor in accordance with the wavelength and intensity of the light form a signal so as to be output to an output section. The image sensor is divided into a CCD (Charge Coupled Device) image sensor and a CMOS (Complementary Metal Oxide Semiconductor) image sensor.
The CCD image sensor is composed of a photodiode as a receiving section, a charge transmitting section, and a signal output section. The photodiode receives light so as to generate a signal charge, the charge transmitting section transmits the signal charge generated by the photodiode to the signal output section by using a CCD without any loss, and the signal output section stores signal charges and detects a voltage proportional to the amount of signal charges so as to produce an analog output. In the CCD image sensor, since the signal charges are converted into a voltage at the last stage, the noise characteristic is excellent. Accordingly, the CCD image sensor is used in a high-definition digital camera, camcorder or the like. In the CCD image sensor, however, a large voltage is required because of a complicated driving method thereof, and power consumption is large because a separate driving circuit is needed. Further, a signal processing circuit cannot be implemented within a CCD chip because the number of mask processes is large. Accordingly, in order to overcome such drawbacks, the development of a submicron CMOS image sensor is being actively performed.
Different from the CCD image sensor, a CMOS image sensor converts signal charges generated by each photodiode into a voltage and transmits the converted voltage to the last stage. Therefore, in the CMOS image sensor, the signal thereof is weaker than that of the CCD image sensor, and noise not only occurs regularly but also occurs due to a dark current. However, as a semiconductor processing technology develops, a CDS (Correlated Double Sampling) circuit is adopted to significantly reduce reset noise, so that an improved image signal can be obtained. In other words, the CDS circuit samples a reset voltage of an image pixel and then samples a signal voltage. At this time, an output of the CDS circuit equals the difference between the reset voltage and the signal voltage. Thus, the CDS circuit may reduce fixed pattern noises due to threshold voltage differences of the transistors in image pixels as well as the reset noises due to the reset voltage differences, thereby obtaining a higher resolution image. Therefore, the CMOS image sensor is widely used in a digital camera, a mobile phone, a PC camera, and the like. Further, the use of the CMOS image sensor is expanded to an automobile.
On the other hand, in order to implement such an image sensor used in an automobile, a number of requirements should be satisfied to obtain a high-resolution image. That is, a high signal-to-noise ratio (S/N), high quantum efficiency, a high fill factor, a high dynamic range and the like are should be satisfied.
FIG. 1 is a cross-sectional view illustrating a unit pixel 100 of a CMOS image sensor according to the related art, and FIG. 2 is a plan view illustrating a color filter array 110 according to the related art.
As shown in FIG. 1, the unit pixel 100 of the conventional CMOS image sensor is composed of a photodiode 120 which detects a light signal to convert into a current signal, an active pixel circuit 130 which converts the converted current signal into a voltage signal, and a color filter which transmits a light signal on the photodiode 120 and the active pixel circuit 130.
The active pixel array according to the related art is where the unit pixels are arranged. The active pixel array is composed of a pixel sensor array, a color filter array, and an insulating layer. The pixel sensor array is formed by repeatedly arranging a basic unit which is set to a 2×2 array structure composed of two pixel sensors, which detect a green light signal to convert into a voltage signal, and two pixel sensors which detect a red or blue light signal to convert into a voltage signal. The color filter array is formed on the pixel sensor array and is formed by repeatedly arranging a basic unit which is set to a 2×2 array structure composed of two filters, which transmit a green light signal, and two filters which transmit red and blue light signals. The insulating layer is formed between the pixel sensor array and the color filter array and on the color filter.
As shown in FIG. 2, the color filter array 110 is formed by repeatedly arranging a basic unit 200 which is set to a 2×2 array structure composed of first and third filers 200a and 200c which transmit a green light signal, a second filter 200b which transmits a red light signal, and a fourth filter 200d which transmits a blue light signal.
FIGS. 3A and 3B show an active pixel circuit 130 according to the related art. FIG. 3A shows a 3-transistor type active pixel circuit 130, and FIG. 3B shows a 4-transistor type active circuit 130.
As shown in FIG. 3A, the 3-transistor type active pixel circuit 130 is composed of a first switch 130a which changes the potential of a node by stored signal charges so as to change a bias, a second switch 130b which is connected the first switch 130a and receives a row select signal so as to output the voltage signal converted by the photodiode 120 to a column line, and a third switch 130c which receives a reset signal so as to reset the stored signal charges. The first to third switches 130a to 130c are composed of transistors.
As shown in FIG. 3B, the 4-transistor type active pixel circuit 130 is composed of a first switch 130a which changes the potential of a node by stored signal charges so as to change a bias, a second switch 130b which is connected to the first switch 130a and receives a row select signal so as to output the voltage signal converted by the photodiode 120 to a column line, a third switch 130c which receives a reset signal so as to reset the stored signal charges, and a fourth switch 130d which receives a transfer signal T so as to transfer the signal charges generated by the photodiode 120. The first to fourth switches 130a to 130d are composed of transistors.
The active pixel circuits 130 shown in FIGS. 3A and 3B respectively have an advantage. The 3-transistor type active pixel circuit 130 shown in FIG. 3A maintains a high fill factor because 3 transistors are used, but the noise performance is low. The 4-transistor type active pixel circuit 130 has more excellent noise performance than the 3-transistor type active pixel circuit 130, but maintains a low fill factor because four transistors are used.
FIG. 4 is a circuit diagram showing a basic unit 400 of the conventional pixel sensor array, which is constructed by using the 4-transistor type active pixel circuit.
As shown in FIG. 4, the basic unit of the conventional pixel sensor array is composed of first to fourth pixel sensors 400a to 400d. The first pixel sensor 400a is composed of a first photodiode 401 which detects a green light signal so as to generate a current signal and a first active pixel circuit 402 which converts the current signal into a voltage signal to output. The second pixel sensor 400b is composed of a second photodiode 403 which detects a red light signal so as to generate a current signal and a second active pixel circuit 404 which converts the current signal into a voltage signal to output. The third pixel sensor 400c is composed of a third photodiode 405 which detects a green light signal so as to generate a current signal and a third active pixel circuit 406 which converts the current signal into a voltage signal to output. The fourth pixel sensor 400d is composed of a fourth photodiode 407 which detects a blue light signal so as to generate a current signal and a fourth active pixel circuit 408 which converts the current signal into a voltage signal to output.
FIG. 5 is a block diagram showing a CMOS image sensor 500. As shown in FIG. 5, the CMOS image sensor 500 includes an active pixel sensor array 501 which is formed by repeatedly arranging the basic unit 400 of the pixel sensor array shown in FIG. 4, a control register 502, a timing and control circuit 503 which control timing and various signals, an analog signal processing section 505 which adjusts an analog signal, and an analog-digital converter 504 which converts an analog signal to a digital signal.
The active pixel sensor array 501 output an analog signal on an image, the analog signal processing section 505 corrects and adjusts the analog signal, and the analog-digital converter converts the analog signal into a digital signal. Then, the digital signal is transmitted to an image signal processor (ISP).
Recently, a light receiving area is narrowed as the pixel size is reduced in accordance with high density. Therefore, an influence of noise increases more and more, so that a signal-to-noise ratio (S/N) and performance of dynamic range are deteriorated.
As a light receiving area is reduced more and more, high resolution cannot be maintained in the related art where only one color is output from one pixel.